Dual gate structure for imagers and method of formation

ABSTRACT

A device, as in an integrated circuit, includes diverse components such as transistors and capacitors. After conductive layers for all types of components are produced, a silicide layer is provided over conductive layers, reducing resistance. The device can be an imager in which pixels in an array includes a capacitor and readout circuitry with NMOS transistors. Periphery circuitry around the array can include PMOS transistors. Because the silicide layer is formed after the conductive layers, it is not exposed to high temperatures and, therefore, migration and cross-contamination of dopants is reduced.

FIELD OF THE INVENTION

The invention relates to gate structures for solid state imagingdevices, including CMOS and CCD imaging devices.

BACKGROUND OF THE INVENTION

In CCD, CMOS and other types of imagers, field effect transistors suchas MOSFETs are employed as part of logic circuitry and DRAM cell region(memory cell region). Conventionally, MOSFETs are fabricated by placingan undoped polycrystalline material, for example polysilicon, over arelatively thin gate oxide, and implanting the polycrystalline materialand adjacent active regions with an impurity dopant material to formsource and drain regions. If the impurity dopant material for formingthe source/drain regions is n-type, then the resulting MOSFET is anNMOSFET (“NMOS”) device. Conversely, if the impurity dopant material forforming the source/drain regions is p-type, then the resulting MOSFET isa PMOSFET (“PMOS”) device.

To prevent current leakage and refresh sensitivity from occurring in thesource/drain region of an embedded DRAM, the resistance of thepolysilicon gate has to be reduced. This reduction in the resistance ofthe polysilicon gate is typically achieved by forming a silicide layer,such as a tungsten silicide layer, on the polysilicon gate to improvethe conductivity of the gate. Although the formation of a silicide layeron the polysilicon gate is highly desirable for embedded DRAMs, thesilicide layers pose some drawbacks that need to be overcome.

To better illustrate the drawbacks as a result of silicide formation forreducing the resistance of the polysilicon gate, reference is now madeto FIG. 1, which is a schematic cross-sectional view of an embedded DRAMhaving a memory cell region 20a formed in memory region A of thesubstrate 10, and a logic circuitry 20 b formed in logic region B of thesubstrate 10.

The structures of FIG. 1 are formed by providing a gate oxide layer 12and an undoped polysilicon layer on the substrate 10. Subsequent to theformation of undoped polysilicon layer, P-type and N-type ions arerespectively implanted into the undoped polysilicon layer by using animplanting mask to form a doped polysilicon layer 13. A silicide layer14, for example a tungsten silicide layer 14, is formed on the dopedpolysilicon layer 13 to improve the conductivity of the gate formed insubsequent processes. The tungsten silicide layer 14, doped polysiliconlayer 13 and gate oxide layer 12 are patterned to form dual gatestructures 16 a, 16 b each having respective N+ polysilicon layer 13 aand P+ polysilicon layer 13 b. Spacers 15 are formed on the sidewalls ofthe dual gate structure 16 a and 16 b. The substrate 10 underneath thedual gate structure 16 a and gate structure 16 b has source/drainregions 17 a, 17 b formed therein. The source/drain regions 17 a, 17 bare formed by implanting ions into the substrate 10. Ions in thesource/drain regions 17 a, 17 b need to be activated thermally or byrapid thermal process (RTP).

As noted above, the formation of the tungsten silicide 14 is desirableas it increases the conductivity of the polysilicon layer 13 a, 13 b. Insome IC circuit, NMOS and PMOS are directly connected to each other.However, due to the high backend thermal budget and because thediffusion coefficient of impurities in the tungsten silicide layer 14 ismuch larger than that in the polysilicon layer 13 a, 13 b, N-type ions(typically phosphorus) in the polysilicon layer 13 a of the dual gate 16a diffuse into the P-type polysilicon layer 13 b. Similarly, P-type ions(typically boron) in the polysilicon layer 13 b of the dual gate 16 bdiffuse into the N-type polysilicon layer 13 a. Therefore,interdiffusion 19 as seen in FIG. 1 is caused between the polysiliconlayers 13 a, 13 b of the dual gate. In addition, P-type ions (typicallyboron) in the polysilicon layer 13 b also diffuse into the semiconductorsubstrate 10 (typically silicon). This migration of impurityboron/phosphorus atoms at the source/drain interface and the diffusionof boron atoms into the silicon substrate decreases the “refresh time”of the DRAM device, and consequently increases the DRAM error rate. The“refresh time” of a DRAM cell is defined as the length of time overwhich the DRAM cell can retain a sufficient amount of charge for itsintended data state to be determined by a sense amplifier circuit.Before this period of time expires, the DRAM cell must be reprogrammedor “refreshed” and, consequently, it is desirable that the refresh timebetween the refresh operations be as long as possible.

The above-noted migration and cross-contamination of impurityboron/phosphorus atoms is further exacerbated by the high temperatureprocesses for the formation of capacitors which are employed inconjunction with other device components for charge storage and/or inanalog signal processing circuits. In some IC designs, the formation ofthese capacitors requires deposition of another layer of polysilicon(employed as capacitor electrode layer) subsequent to the formation ofthe silicide layer described above. The subsequent formation of thispolysilicon layer typically requires a high temperature process of about750° C. to about 800° C. As a result of these high temperaturerequirements for the capacitor formation, the P-type ions (typicallyboron) further diffuse into the silicon substrate and from the P-typepolysilicon layer 13 b into the N-type polysilicon layer 13 a, which inturn makes the N-type polysilicon layer 13 a more like the P-typepolysilicon layer 13 b.

The above-noted migration and cross-contamination are furtherencountered when designing various integrated circuits that includediverse components such as transistors and capacitors. For example, animager IC may include an array of pixels, each including a capacitiveelements that stores photogenerated charge and readout circuitry toprovide a readout signal indicating the level of stored charge. Thereadout circuitry typically includes several transistors, and the imagerIC may also have peripheral circuitry that also includes capacitiveelements and transistors. Each component, whether a capacitor or atransistor, may include one or more conductive layers and a silicidelayer formed over the conductive layer, to reduce resistance.

Accordingly, there is a need for an improved method for preventing themigration of impurity atoms into the active regions of a DRAM device, aswell as a method for increasing the refresh time and reducing the errorrate of such DRAM devices. There is also a need for the formation of asilicide layer that would prevent the occurrence of the above-mentionedproblems. An optimized process for the formation of improved imagers andimaging devices having array and periphery area transistors andcapacitors with improved characteristics is also needed.

SUMMARY OF THE INVENTION

The present invention provides devices, integrated circuits and diversecomponents in which silicide layers reduce the resistance of conductivelayers. In exemplary embodiments, imaging devices include array gatestructures and capacitors and peripheral gate structures, at least oneof these structures including a silicide layer. The invention alsoprovides methods of forming a silicide region for such gate structuressubsequent to the formation of capacitor structures.

These and other features and advantages of the invention will be moreapparent from the following detailed description that is provided inconnection with the accompanying drawings and illustrated exemplaryembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an embedded DRAM as knownin the art.

FIG. 2 is a schematic cross-sectional view of a dual gate structureformed according to an exemplary embodiment of the invention and at aninitial stage of processing.

FIG. 3 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 2.

FIG. 4 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 3.

FIG. 5 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 4.

FIG. 6 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 5.

FIG. 7 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 6.

FIG. 8 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 7.

FIG. 9 is a schematic cross-sectional view of the structure of FIG. 2 ata stage of processing subsequent to that shown in FIG. 8.

FIG. 10 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 9.

FIG. 11 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 10.

FIG. 12 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 11.

FIG. 13 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 12.

FIG. 14 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 13.

FIG. 15 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 14.

FIG. 16 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 15.

FIG. 17 is a schematic cross-sectional view of the structure of FIG. 2at a stage of processing subsequent to that shown in FIG. 16.

FIG. 18 is a schematic cross-view of elements of a CMOS imaging device.

FIG. 19 illustrates a block diagram of a CMOS imager device having apixel array, wherein the imager device may be combined with a processorin a single integrated circuit fabricated according to the presentinvention.

FIG. 20 illustrates a schematic diagram of a computer processor systemwhich may utilize an imaging device, for example, a CMOS imaging deviceconstructed in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousspecific embodiments in which the invention may be practiced. Theseembodiments are described with sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be employed, and that structural and logicalchanges may be made without departing from the spirit or scope of thepresent invention.

The terms “substrate” and “wafer” can be used interchangeably in thefollowing description and may include any semiconductor-based structureor insulating structure on or at the surface if which circuitry may beformed. The structure should be understood to include silicon,silicon-on insulator (SOI), silicon-on-sapphire (SOS), doped and undopedsemiconductors, epitaxial layers of silicon supported by a basesemiconductor foundation, and other semiconductor and insulatingstructures. The semiconductor need not be silicon-based. Thesemiconductor could be silicon-germanium, germanium, or galliumarsenide. When reference is made to the substrate in the followingdescription, previous process steps may have been utilized to formregions or junctions in or over the base semiconductor or foundation.

The term “pixel” refers to a discrete picture element unit cellcontaining a photodetector and transistors or other components forconverting electromagnetic radiation to an electrical signal. Forpurposes of illustration, a representative pixel according to oneembodiment of the invention is illustrated in the figures anddescription herein. An array or combination of pixels together maycomprise a photodetector array for use in a CMOS or CCD imager device.Typically, fabrication of all pixels of a photodetector array willproceed simultaneously in a similar fashion.

Referring now to the drawings, where like elements are designated bylike reference numerals, FIGS. 2-17 illustrate a method for thefabrication of an integrated circuit that includes an NMOSFET 100 (FIGS.16-17), a PMOSFET 200 (FIGS. 16-17) and a capacitor structure 300 (FIGS.15-17). The NMOSFET 100 and PMOSFET 200 have respective gate structuresthat include a silicide region 66 (FIGS. 10-17) formed subsequent to theformation of capacitor structure 300 (FIGS. 15-17), which also includesa silicide region 66. As described in detail below, the NMOSFET 100(FIGS. 16-17) and the PMOSFET 200 (FIGS. 16-17) comprising silicideregion 66 (FIGS. 10-17) formed according to embodiments of the presentinvention have decreased migration and cross-contamination of impurityatoms in their respective gate structures. The decreased migration andcross-contamination of impurity atoms is achieved by forming layers in asequence such that silicide region 66 is not subjected to hightemperatures.

Although the present invention will be described below with reference tothe formation of an NMOS transistor and a PMOS transistor each in theirrespective array and periphery regions of a CMOS imager IC, such as theNMOSFET 100 (FIGS. 16-17) and the PMOSFET 200 (FIGS. 16-17), it must beunderstood that the invention is not limited to this embodiment.Accordingly, the invention also contemplates the formation of PMOStransistors in both the array and periphery regions of a CMOS imager ICor other IC, the formation of NMOS transistors in both the array andperiphery regions of a CMOS imager IC or other IC, as well as theformation of a plurality of MOS transistors of the same or differentconductivity type. In addition, although the NMOSFET 100 (FIGS. 16-17)and the PMOSFET 200 (FIGS. 16-17) are illustrated as being adjacent inthe drawings, in actual practice this will likely not occur, as NMOSFET100 is formed in an array area of a CMOS imager IC and PMOSFET 200 isformed in a peripheral area outside the array area. Thus, the belowillustrated and described embodiments are only exemplary, and thepresent invention is not limited to the illustrated embodiments.

FIG. 2 illustrates a semiconductor substrate base 50, for example, adoped silicon substrate 50 having an active dopant concentration withinthe range of about 1×10¹⁶ to 1×10¹⁸ atoms per cm³, more preferably about5×10¹⁶ to 5×10¹⁷ atoms per cm³. As also shown in FIG. 2, isolationregions 51 are formed in the substrate 50 by any known technique, suchas thermal oxidation of the underlying silicon in a LOCOS process, or byetching trenches and filling them with a dielectric or an oxide materialin an STI process. This way, the isolation regions 51 (FIG. 2) may befield oxide regions (FOX) or shallow trenches for isolation (STI).

According to an exemplary embodiment of the present invention, isolationregions 51 (FIG. 2) are shallow trenches for isolation filled with highdensity plasma (HDP) oxide, a material which has a high ability toeffectively fill narrow trenches. Alternatively, an insulating layer(not shown) formed of an oxide or of silicon nitride, for example, maybe formed on the trench sidewalls, prior to filling the trench with theisolation dielectric, to aid in smoothing out the corners in the bottomof the trench and to reduce the amount of stress in the dielectric usedto later fill in the trench.

As known in the art, isolation regions 51 provide electrical andphysical separation, as well as isolation between neighboring transistorregions on the semiconductor substrate 50. As illustrated in FIG. 2, themiddle isolation region 51 isolates region A of the semiconductorsubstrate 50 on which array circuitry will be subsequently formed, fromregion B of the semiconductor substrate 50 on which peripheral circuitrywill be subsequently formed, all according to embodiments of the presentinvention.

Subsequent to the formation of isolation regions 51 (FIG. 2), a thingate oxide layer 56, which will act as a gate insulator layer, is formedover the semiconductor substrate 50, as also illustrated in FIG. 2. Thethin gate oxide layer 56 may comprise silicon dioxide (SiO₂), forexample, which may be thermally grown in an oxygen ambient, at atemperature between about 600° C. to about 1000° C. and to a thicknessof about 10 Angstroms to about 100 Angstroms. The gate insulator is notlimited to silicon oxide and other dielectric materials such asoxynitride, Al₂O₃, Ta₂O₅ or other high k material may be used as gateinsulator layer.

As illustrated in FIG. 3, an undoped gate layer 58 is formed over thethin gate oxide layer 56. According to an embodiment of the presentinvention, the undoped gate layer 58 is formed of polysilicon which maybe deposited over the thin gate oxide layer 56 by, for example, a lowplasma chemical vapor deposition (LPCVD) method at a temperature ofabout 300° C. to about 700° C. and to a thickness of about 100 Angstromsto about 2,000 Angstroms.

According to another embodiment of the present invention, the undopedgate layer 58 is a layer of silicon/germanium (Si/Ge) material.According to this embodiment, the gate silicon/germanium layer is grownby a low plasma chemical vapor deposition (LPCVD) method in a reactionchamber at a temperature of about 900° C. to about 1200° C., and byemploying a silicon (Si) and germanium (Ge) gas source to introduce agaseous species containing silicon and germanium in the same reactionchamber. For example, the silicon gas source may be, for example, silane(SiH₄), higher order silanes, such as disilane (Si₂H₆), as well as othergaseous sources of silicon, such as dichlorsilane (SiH₂Cl₂),trichlorsilane (SiHCl₃), or tetrachlorsilane (SiCl₄). The germanium gassource may be, for example, a germane (GeH₄) source, or any othergaseous source containing germanium. The combination of the two gaseoussources allows deposition of the gate silicon/germanium layer to athickness of about 100 Angstroms to about 1500 Angstroms, morepreferably of about 500 Angstroms to about 900 Angstroms. Although, asdescribed above, the undoped gate layer 58 may be formed of varioussemiconductor materials, reference to the undoped gate layer 58 will bemade in this application as to the undoped polysilicon layer 58.

Subsequent to the formation of the undoped polysilicon layer 58 (FIG.3), a first patterned photoresist layer 59 b (FIG. 4) is formed to athickness of about 1,000 Angstroms to about 10,000 Angstroms over theundoped polysilicon layer 58 and over the region B, but not over theregion A on which an array of pixel cells or other cells with capacitivecomponents and transistors will be subsequently formed. As shown in FIG.4, the first patterned photoresist layer 59 b is also not formed overthe middle STI region 51, over which capacitor structure 300 (FIGS.15-17) will be formed according to embodiments of the present invention.Thus using the thick patterned photoresist layer 59 b as a mask, dopantor impurity atoms of a first conductivity type are introduced into theexposed portion of the undoped polysilicon layer 58 located above theregion A of the substrate 50 and above the middle STI region 51 by usinga first ion implantation 53 (FIG. 4) to form a doped polysilicon layer60 a, illustrated in FIG. 5.

According to an exemplary embodiment of the invention, the dopant orimpurity atoms of the first conductivity type are of n-type conductivityfor NMOS device fabrication. Thus, n-type dopants such as phosphorous(P), arsenic (As) or antimony (Sb) are implanted into the exposed regionof the undoped polysilicon layer 58 by using the first ion implantation53 (FIG. 4) to form the n+ doped polysilicon layer 60 a (FIG. 5). Ionimplantation is performed by placing the substrate 50 in an ionimplanter, for example, and implanting appropriate dopant ions into theexposed portion of the undoped polysilicon layer 58. For example,phosphorous is introduced into the exposed portion of the undopedpolysilicon layer 58 of FIG. 4 using ion implantation at an acceleratedenergy of about 1 keV to about 50 keV and a dose concentration of about1×10¹⁵/cm² to about 5×10¹⁵/cm². A heat treatment such as an annealtreatment at about 600° C. to about 1000° C., may be optionally employedto activate the dopant within the n+ doped polysilicon layer 60 a ofFIG. 5.

Subsequent to the formation of the n+ polysilicon layer 60 a of FIG. 5,the substrate 50 may optionally undergo another ion implantation usingthe first patterned photoresist layer 59 b as a mask (or anotherphotoresist layer as a mask), this time with dopant impurity ions of asecond conductivity type. In this manner, p-type dopant impurity ions,such as boron (B), boron fluoride (BF₂) or indium (In) are implantedinto the substrate 50 to form a p-type well 62 a as shown in FIG. 5. Ionimplantation is performed by placing the substrate 50 in an ionimplanter, for example, and implanting appropriate p-type dopant ionsinto region A of the substrate 50. For example, boron is introduced intoregion A using ion implantation at an accelerated energy of about 15 keVto about 150 keV and a dose concentration of about 1×10¹¹/cm² to about5×10¹³/cm². A heat treatment, for example, an anneal treatment at about600° C. to about 1000° C., may be optionally employed to activate thedopant within the doped p-type well 62 a of FIG. 5.

Although the invention has been described above with reference to theformation of the p well 62 a (FIG. 5) subsequent to the formation of then+ polysilicon layer 60 a (FIG. 5), the invention is not limited to thisexemplary embodiment. As such, the invention also contemplates theformation of the p well 62 a first, and then followed by the formationof the n+ polysilicon layer 60 a.

Referring now to FIG. 6, a second patterned photoresist layer 59 a isformed to a thickness of about 1,000 Angstroms to about 10,000 Angstromsover the n+ doped polysilicon layer 60 a and the p doped well 62 a, butnot over the region B on which periphery circuitry will be subsequentlyformed according to a method of the present invention. As alsoillustrated in FIG. 6, the second patterned photoresist layer 59 a isalso formed over the middle STI region 51. Accordingly, using the thickpatterned photoresist layer 59 a as a mask, dopant or impurity atoms ofthe second conductivity type are introduced into the exposed portion ofthe undoped polysilicon layer 58 located above the region B of thesubstrate 50 to form a p+ doped polysilicon layer 60 b, as illustratedin FIG. 7.

P-type dopant impurity ions, such as boron (B), boron fluoride (BF₂) orindium (In) are implanted into the exposed region of the undopedpolysilicon layer 58 of FIG. 6 by using a second ion implantation 55(FIG. 6) to form the p+ doped polysilicon layer 60 b (FIG. 7). Ionimplantation is performed by placing the substrate 50 in an ionimplanter, and implanting appropriate dopant ions into the exposedportion of the undoped polysilicon layer 58. For example, boron isintroduced into the exposed portion of the undoped polysilicon layer 58of FIG. 6 using ion implantation at an accelerated energy of about 1 keVto about 50 keV and a dose concentration of about 1×10¹⁵/cm² to about5×10¹⁵/cm². A heat treatment, for example, an anneal treatment at about600° C. to about 1000° C., may be optionally employed to activate thedopant within the p+ doped polysilicon layer 60 b of FIG. 7.

According to an embodiment of the invention and subsequent to theformation of the p+ polysilicon layer 60 b of FIG. 7, the substrate 50may optionally undergo another ion implantation to form a lightly dopedor very lightly doped n-type well 62 b illustrated in FIG. 7. For this,n-type dopants such as phosphorous (P), arsenic (As) or antimony (Sb)are implanted into the region B of the substrate 50 to form the n-typewell 62 b. Although the invention has been described above withreference to the formation of the n-type well 62 b (FIG. 7) subsequentto the formation of the p+ polysilicon layer 60 b (FIG. 7), theinvention is not limited to this exemplary embodiment. As such, theinvention also contemplates the formation of the n-type well 62 b first,and then followed by the formation of the p+ polysilicon layer 60 b. Theinvention could also be embodied in structures without a well.

Referring now to FIG. 8 and in accordance with an exemplary embodimentof the present invention, a dielectric layer 63 is formed over the dopedpolysilicon layers 60 a, 60 b, as shown in FIG. 8. The dielectric layer63 may be formed of an oxide, nitride, Al₂O₃, Ta₂O₅, or BST material, orany other nonconductor of direct electric current. The dielectric layer63 may be formed by any suitable deposition technique, includingchemical vapor deposition (CVD) techniques such as low pressure chemicalvapor deposition (LPCVD) or high density plasma (HDP) deposition. Thedielectric layer 63 may be formed to a thickness of about 10 Angstromsto about 2,000 Angstroms, and with any desired shape and geometrysuitable to the dielectric of a capacitor.

Subsequent to the formation of the dielectric layer 63, a conductivelayer 64 of conductive material that will eventually form a secondcapacitor electrode of capacitor 300 (FIGS. 15-17) is formed over thedielectric layer 63, as also shown in FIG. 8. The conductive layer 64may be formed of any suitable electrode material, including but notlimited to metals, metal alloys, conductive metal oxides or combinationsof such metals, metal alloys and conductive metal oxides. The conductivelayer 64 may be also formed of doped polysilicon, or conductivecombinations of polysilicon and other metals and compositions, such aspolysilicon/HSG (hemispherical grained polysilicon), polysilicon/WSi andpolysilicon/WN/W, among others.

If the conductive layer 64 comprises doped polysilicon, then thepolysilicon may be formed by deposition, for example CVD, PECVD andLPCVD, at a temperature between about 600° C. to about 800° C., to athickness of about 500 Angstroms to about 2,000 Angstroms. Theconductive layer 64 may be optionally planarized after its formation,using any suitable planarizing technique.

Reference is now made to FIG. 9. The conductive layer 64 and thedielectric layer 63 are patterned with a photoresist layer and a maskand then anisotropically etched through the patterned photoresist, toobtain capacitor stack 65 located above the n+ polysilicon layer 60 aand the middle STI region 51 of FIG. 9.

Subsequent to the formation of capacitor stack 65 of FIG. 9, a silicidelayer 66 and an insulating layer 67 are sequentially formed over thecapacitor stack 65, the n+ polysilicon layer 60 a and the p+ polysiliconlayer 60 b, as shown in FIG. 10. The silicide layer 66 may formed bydepositing a layer of metal capable of forming a silicide, such ascobalt, nickel, molybdenum or titanium, for example, over the dopedpolysilicon layers 60 a, 60 b (FIG. 9) and over the capacitor stack 65(FIG. 9) by sputtering by RF or DC, or by other similar methods such asCVD, to a thickness of about 100 Angstroms to about 800 Angstroms.Subsequent to the deposition of the metal capable of forming a silicide,the substrate 50 is subjected to a rapid thermal anneal (RTA), typicallyfor about 10 to 60 seconds, using a nitrogen ambient at about 600° C. toabout 850° C., so that the metal in direct contact with the dopedpolysilicon layers 60 a, 60 b and the conductive layer 64 is convertedto its silicide. As shown in FIG. 16, silicide layer 66 forms conductiveregions 66 on top of the doped polysilicon layers 60 a, 60 b.Preferably, the refractory metal has low resistance and low resistivityas a silicide. However, the refractory metal silicide may comprise anyrefractory metal, including but not limiting to tungsten, titanium,cobalt, tantalum, molybdenum, and platinum. The refractory metalsilicide may also comprise combination of silicides doped with nitrogen,such as tungsten nitride silicide or a combination of tungsten nitridesilicide/tungsten silicide, for example.

Insulating layer 67, which is preferably a cap material formed over thesilicide regions 66, is also illustrated in FIG. 10. The cap materialmay be formed of silicon dielectrics such as silicon nitride or siliconoxide, but TEOS or carbides may be used also. This layer may be formedvia PECVD and LPCVD deposition procedures, for example, at a temperaturebetween about 300° C. to about 1000° C., to a thickness of about 500Angstroms to about 2,000 Angstroms.

Next, the structure of FIG. 10 is patterned with a photoresist layer 69(FIG. 11) and a mask which are both formed over the cap material 67. Inthis manner, the silicide layer 66 and the cap material 67, but not thedoped polysilicon layers 60 a, 60 b, are anisotropically etched throughthe patterned photoresist, to obtain the structure of FIG. 12.

Subsequent to the removal of the remaining photoresist layer 69 fromatop the cap material 67, another photoresist layer 79 is formed overthe cap material 67 and over the exposed portions of the conductivelayer 64 and doped polysilicon layers 60 a, 60 b, as shown in FIG. 13.Although, for simplicity, FIG. 13 illustrates the formation of thephotoresist layer 79 over the cap material 67 and over the exposedportions of the doped polysilicon layers 60 a, 60 b in a schematicmanner, a person skilled in the art will understand that the photoresistlayer 79 is conformally deposited over the structures of FIG. 12.

Subsequent to the formation of the photoresist layer 79, the structureof FIG. 13 is subjected to another photolithographic patterning and etchprocess to etch through the doped polysilicon layers 60 a, 60 b andthrough the thin gate oxide layer 56 to form the structure of FIG. 14.Upon removal of the remaining portion of the photoresist layer 79 fromthe top of the cap material 67, the resultant structures (FIG. 15) aregate stacks 70 a, 70 b and capacitor structure 300 formed over thesemiconductor substrate 50. Each of the gate stacks 70 a, 70 b comprisesgate oxide layer 56, one of the doped polysilicon layer 60 a, 60 b, thesilicide region 66 and the nitride cap 67. Gate 70 a comprises the n+doped polysilicon layer 60 a, while gate 70 b comprises the p+ dopedpolysilicon layer 60 b. Capacitor structure 300 comprises dopedpolysilicon layer 60 a, which acts as a first capacitor electrode,dielectric layer 63 and conductive layer 64, which acts as a secondcapacitor electrode. Also shown as part of the capacitor structure 300are silicide region 66 and nitride cap 67.

For each of the gate stacks 70 a, 70 b, silicide region 66 has beenformed subsequent to the high temperature deposition of the conductivelayer 64, which forms the second capacitor electrode 64 of capacitor300. Thus, as the silicide region 66 is not subjected to hightemperature processes, the diffusion coefficient of impurities in thesilicide layer 66 is reduced and, accordingly, N-type ions (e.g., boron)in the polysilicon layer 60 a of the gate 70 a do not diffuse into theP-type polysilicon layer 60 b. Similarly, P-type ions (e.g., phosphorus)in the polysilicon layer 60 b of the gate 70 b do not diffuse into theN-type polysilicon layer 60 a. Therefore, no interdiffusion takes placebetween the polysilicon layers 60 a, 60 b of the gate structures. Inaddition, the diffusion of N-type ions (e.g., boron) of the polysiliconlayer 60 a into the semiconductor substrate 50 (e.g., silicon) is alsodecreased.

The gate stacks 70 a, 70 b of FIG. 15 may now be used in a self-alignedimplant process where the gate stacks mask the dopant implantation ofsource/drain regions 73 a, 73 b (FIG. 16) of the adjacent transistorsdefined by the gate stacks. As such, FIG. 15 illustrates the formationof lightly doped drain (LDD) regions 71 a, 71 b in the substrate 50. Asknown in the art, the LDD regions 71 a, 71 b may be formed by implantinglow dosages of conductivity-altering dopants with an LDD mask (notshown) and using the gate stacks 70 a, 70 b as an implantation mask.Thus, in the well 62 a of the region A of the substrate 50, n-typedopants such as arsenic or phosphorous may be implanted at a low energydose, for example of about 2×10¹⁴ atoms/cm², using the gate stack 70 aas a mask. Similarly, in the well 62 b of the region B of the substrate50, n-type dopants such as arsenic or phosphorous may be also implantedat a low energy dose, for example of about 2×10⁴ atoms/cm², using thegate stack 70 b as a mask.

The next step in the process flow is the formation of spacers 74 and 75a, 75 b, 75 c, all illustrated in FIG. 16. For example, spacers 74 maybe formed by blanket depositing a silicon nitride film or a siliconoxide material over the structure of FIG. 15 and then anisotropicallyetching it with an RIE plasma to form the spacers 74 on each of thesidewalls of the gate stacks 70 a, 70 b.

The gate stacks 70 a, 70 b protected by spacers 74 can now undergoprocessing steps for the formation of source/drain regions in thesubstrate 50. As such, using the gate stacks 70 a, 70 b as animplantation mask, heavily doped source/drain regions 73 a, 73 b areformed in the uncovered portions of the substrate 50 via an ionimplantation procedure performed at an energy of about 3 KeV to about 50keV, and at a dose of about 1×10¹⁵ to about 5×10¹⁵ atoms/cm², using ann-type dopant such as arsenic or phosphorous, for example, and a p-typedopant such as a boron or indium, for example. Thermal annealing foractivation may be carried out in a nitrogen atmosphere at about 850° C.to about 1050° C. and for about 10 seconds to about 10 minutes.

This way, source and drain regions 73 a, 73 b (FIG. 16), which areheavily doped with n-type and p-type impurity ions, respectively, areformed within the wells 62 a, 62 b. Thus, an n-p-n type (NMOS)transistor 100 (FIG. 16) and a p-n-p type (PMOS) transistor 200 (FIG.16) are formed of the gate stacks 70 a, 70 b and their respective n-typesource/drain region 73 a and p-type source/drain region 73 b within thewells 62 a, 62 b. NMOS transistor 100 is formed in the memory array areaand PMOS transistor 200 is formed in the periphery area. Additionalimplants may be conducted into the substrate 50 to form a photodiode orother photoconversion devices.

Examples of the implementation of the present invention and of thecharacteristics of the NMOS and PMOS transistors comprising silicideregions formed subsequent to the formation of capacitor structuresaccording to embodiments of the present invention will now be describedwith reference to FIGS. 17-20. The NMOS transistor 100 (FIG. 16) may beused as an access transistor in a memory device such as a random accessmemory device, in flash memory, SRAM memory or CMOS imagers, amongothers. For example, the NMOS transistor 100 (FIG. 16) may be used in aDRAM memory device formed, as known in the art, of a pair of memorycells comprising respective access NMOS transistors, at least one beingNMOS transistor 100 formed according to the present invention. FIG. 17depicts contacts 88, all formed within one or more insulating layers,such as insulating layer 80, formed over the NMOS transistor 100, thePMOS transistor 200 and the capacitor 300. Insulating layer 80 may be alayer of tetraethyl orthosilicate (TEOS) formed by conventionaldeposition processes, for example thermal oxidation or chemical vapordeposition (CVD). Insulating layer 80 may be optionally formed as anitride, oxide, ON (oxide-nitride), NO (nitride-oxide), ONO(oxide-nitride-oxide), or other insulating material.

The NMOS or PMOS transistor 100, 200 (FIGS. 16-17) may be employed inany integrated circuit structures, for example, as part of, or inconjunction with, various elements of a four-transistor (4T) pixel cell400 of a CMOS imaging device, as shown schematically in FIG. 18.

FIG. 18 depicts transfer transistor 91, reset transistor 92, sourcefollower transistor 93 and row select transistor 94 as part of the pixelcell 400. Each of these transistors can be an NMOS device, fabricatedlike the NMOS transistor 100 of FIG. 17. Details of the functions andattributes of the these CMOS imaging elements such as transistors 91,92, 93 and 94 are generally known in the art and they are discussed in amultitude of references such as, for example, Nixon et al., “256×256CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-StateCircuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOSActive Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol.41(3), pp. 452-453 (1994), the disclosures of which are incorporated byreference herein.

As known in the art, a CMOS imager circuit includes a focal plane arrayof pixel cells, each one of the cells including either a photodiode, aphotogate, a photodetector or a photoconductor overlying a doped regionof a substrate for accumulating photo-generated charge in the underlyingportion of the substrate. In a CMOS imager, the active elements of apixel cell perform the necessary functions of: (1) photon to chargeconversion; (2) accumulation of image charge; (3) transfer of charge toa floating diffusion node accompanied by charge amplification; (4)resetting the floating diffusion node to a known state before thetransfer of charge to it; (5) selection of a pixel for readout; and (6)output and amplification of a signal representing pixel charge. Thecharge at the floating diffusion node is typically converted to a pixeloutput voltage by a source follower output transistor. Thephotosensitive element of a CMOS imager pixel is typically either adepleted p-n junction photodiode or a field induced depletion regionbeneath a photogate.

For example, photosensor 90 of pixel cell 400 of FIG. 18 may be formedas a photodiode for accumulating photo-generated charge in an underlyingportion of the substrate 50. Photosensor 90 may include, for example, aphotosensitive p-n-p junction region formed at or beneath the uppersurface of substrate 50 by conventional techniques. It should beunderstood that the imagers of the invention may include a photogate,photoconductor, or other image to charge converting device, in lieu of aphotodiode, as the initial accumulator for photo-generated charge.Photosensor 90 may be formed at or beneath the upper surface ofsubstrate 50, and may also be constructed in any arrangement,orientation, shape and geometry, to be integrated with other componentsof a semiconductor device.

As also shown in FIG. 18, capacitor 300 is connected to store chargereceived from photosensor 90 through transfer transistor 91 in responseto a pulse in a transfer (Tx) signal. Previously, capacitor 300 is resetto an uncharged state through reset transistor 92 in response to a resetsignal pulse. The charge stored by capacitor 300 controls conductivityof source follower transistor 93, which provides an output signalindicating its conductivity through row select transistor 94 in responseto a row select signal. The output signal is conducted by column readoutline 95 out of the array of the pixel cells to peripheral readoutcircuitry. Load transistor 96, controlled by voltage Vin, is part of theperipheral circuitry and can be a PMOS transistor fabricated like thePMOS transistor 200 of FIG. 17.

FIG. 19 illustrates a block diagram of a CMOS imager device IC 808having a pixel array 800 containing a plurality of pixels arranged inrows and columns. Each pixel can include pixel cell 400 illustrated inFIG. 18. The pixels of each row in array 800 are all turned on at thesame time by a row select line, and the pixels of each column areselectively output by respective column select lines. The row lines areselectively activated by a row driver 810 in response to row addressdecoder 820. The column select lines are selectively activated by acolumn selector 860 in response to column address decoder 870. The pixelarray is operated by the timing and control circuit 850, which controlsaddress decoders 820, 870 for selecting the appropriate row and columnlines for pixel signal readout. The pixel column signals, whichtypically include a pixel reset signal (V_(rst)) and a pixel imagesignal (V_(sig)), are read by a sample and hold circuit 881 associatedwith the column selector 860. A differential signal (V_(rst)−V_(sig)) isproduced by differential amplifier 862 for each pixel which is amplifiedand digitized by analog to digital converter 875 (ADC). The analog todigital converter 875 supplies the digitized pixel signals to an imageprocessor 880 which forms a digital image.

If desired, the imaging device 808 described above with respect to FIG.19 may be combined with a processor in a single integrated circuit. FIG.20 illustrates an exemplary processing system 900 which may utilize animaging device, for example, a CMOS imager 808 incorporating elementsconstructed in accordance with embodiments of the invention illustratedin FIGS. 2-17. Any one of the electronic components shown in FIG. 20,including CPU 901, may be fabricated as an integrated circuit for use inprocessing images formed in accordance with the imager and methods ofthe present invention.

As illustrated in FIG. 20, the processing system 900 includes one ormore processors 901 coupled to a local bus 904. A memory controller 902and a primary bus bridge 903 are also coupled the local bus 904. Theprocessing system 900 may include multiple memory controllers 902 and/ormultiple primary bus bridges 903. The memory controller 902 and theprimary bus bridge 903 may be integrated as a single device 906.

The memory controller 902 is also coupled to one or more memory buses907. Each memory bus accepts memory components 908 which include atleast one memory device 100. The memory components 908 may be a memorycard or a memory module. Examples of memory modules include singleinline memory modules (SIMMs) and dual inline memory modules (DIMMs).The memory components 908 may include one or more additional devices909. For example, in a SIMM or DIMM, the additional device 909 might bea configuration memory, such as a serial presence detect (SPD) memory.The memory controller 902 may also be coupled to a cache memory 905. Thecache memory 905 may be the only cache memory in the processing system.Alternatively, other devices, for example, processors 901 may alsoinclude cache memories, which may form a cache hierarchy with cachememory 905. If the processing system 900 includes peripherals orcontrollers which are bus masters or which support direct memory access(DMA), the memory controller 902 may implement a cache coherencyprotocol. If the memory controller 902 is coupled to a plurality ofmemory buses 907, each memory bus 907 may be operated in parallel, ordifferent address ranges may be mapped to different memory buses 907.

The primary bus bridge 903 is coupled to at least one peripheral bus910. Various devices, such as peripherals or additional bus bridgesmay-be coupled to the peripheral bus 910. These devices may include astorage controller 911, an miscellaneous I/O device 914, a secondary busbridge 915, a multimedia processor 918, and an legacy device interface920. The primary bus bridge 903 may also be coupled to one or morespecial purpose high speed ports 922. In a personal computer, forexample, the special purpose port might be the Accelerated Graphics Port(AGP), used to couple a high performance video card to the processingsystem 900.

The storage controller 911 couples one or more storage devices 913, viaa storage bus 912, to the peripheral bus 910. For example, the storagecontroller 911 may be a SCSI controller and storage devices 913 may beSCSI discs. The I/O device 914 may be any sort of peripheral. Forexample, the I/O device 914 may be an local area network interface, suchas an Ethernet card. The secondary bus bridge may be used to interfaceadditional devices via another bus to the processing system. Forexample, the secondary bus bridge may be an universal serial port (USB)controller used to couple USB devices 917 via to the processing system900. The multimedia processor 918 may be a sound card, a video capturecard, or any other type of media interface, which may also be coupled toone additional devices such as speakers 919. The legacy device interface920 is used to couple legacy devices, for example, older styledkeyboards and mice, to the processing system 900.

The processing system 900 illustrated in FIG. 20 is only an exemplaryprocessing system with which the invention may be used. While FIG. 20illustrates a processing architecture especially suitable for a generalpurpose computer, such as a workstation, it should be recognized thatwell known modifications can be made to configure the processing system900 to become more suitable for use in a variety of applications. Forexample, many electronic devices which require processing may beimplemented using a simpler architecture which relies on a CPU 901coupled to memory components 908 and/or memory devices 100. Theseelectronic devices may include, but are not limited to audio/videoprocessors and recorders, and digital cameras and/or recorders. The CMOSimager IC 808, when coupled to a pixel processor, for example, may beimplemented in digital cameras and video processors and recorders.Modifications may include, for example, elimination of unnecessarycomponents, addition of specialized devices or circuits, and/orintegration of a plurality of devices.

While the invention is preferably directed to methods for forming imagerdevices with distinct pixel capacitors and periphery capacitors, andstructures incorporating such pixel capacitors and periphery capacitors,one skilled in the art will recognize that the invention can be used toform any type of imager device for integration with one or moreprocessing components in a semiconductor device. For example, althoughthe invention is described above for use in a CMOS image sensor, theinvention is not limited to such and may be used in any suitable imagesensor, for example, CCD image sensors.

The last (output) stage of a CCD image sensor provides sequential pixelsignals as output signals, and uses a floating diffusion node, sourcefollower transistor, and reset gate in a similar manner to the way theseelements are used in the pixel of a CMOS imager. Accordingly, the pixelsformed as described above may be employed in CCD image sensors as wellas CMOS image sensors. The imager devices of the present invention mayalso be formed as different size megapixel imagers, for example imagershaving arrays in the range of about 0.1 megapixels to about 20megapixels.

Although the exemplary embodiments of the present invention have beendescribed for only one NMOS transistor 100 (FIGS. 2-17) formed in thearray area, and for only one PMOS transistor 200 (FIGS. 2-17) formed inthe periphery area, it must be understood that the present inventioncontemplates the formation of a plurality of such MOSFET devices intheir respective area. In addition, although the present invention hasbeen described above with reference to the formation of one NMOStransistor 100 (FIGS. 2-17) in the array area, and one PMOS transistor200 (FIGS. 2-17) in the periphery area, the invention also contemplatesthe formation of PMOS transistors in the array area and NMOS transistorsin the periphery area. Similarly, the scope of the invention includesforming capacitors in both an array and its periphery.

Although the invention has been described above in connection with afour-transistor (4T) pixel cell employing a transfer transistor having atransfer gate, the invention may also be incorporated into athree-transistor (3T) cell, a five-transistor (5T) cell, asix-transistor (6T) cell or a seven-transistor (7T) cell, among others.As known in the art, a 3T cell differs from the 4T cell by the omissionof the charge transfer transistor and associated gate, and the couplingof the n regions of the photodiode and the floating diffusion regionsthrough an overlap of the two or an n region bridging the two, which iswell known in the art. A 5T cell differs from the 4T cell by theaddition of a shutter transistor or a CMOS photogate transistor.

Further, although the invention has been described above with referenceto the formation of transistors and capacitors as part of a CMOS imager,the invention has equal applicability to the formation of transistorsand capacitors as part of a CCD imager, a global shutter transistor, ahigh dynamic range transistor or a storage gate, among others.

The above description illustrates preferred embodiments that achieve thefeatures and advantages of the present invention. It is not intendedthat the present invention be limited to the illustrated embodiments.Modifications and substitutions to specific process conditions andstructures can be made without departing from the spirit and scope ofthe present invention. Accordingly, the invention is not to beconsidered as being limited by the foregoing description and drawings,but is only limited by the scope of the appended claims.

1. A method of forming a semiconductor structure, comprising the stepsof: providing a conductive layer over a semiconductor substrate; formingat least one capacitor electrode adjacent said conductive layer; andsubsequently forming a silicide layer over at least said conductivelayer.
 2. The method of claim 1, wherein said silicide layer is formedover said conductive layer and said capacitor electrode.
 3. The methodof claim 1, further comprising the step of providing a dielectric layerover said conductive layer.
 4. The method of claim 3, wherein said stepof forming at least one capacitor electrode comprises forming apolysilicon layer over said dielectric layer.
 5. The method of claim 4,wherein said polysilicon layer is formed by deposition at a temperaturebetween about 600° C. to about 800° C.
 6. The method of claim 1, whereinsaid step of forming a silicide layer includes providing a metal layerover at least said conductive layer and annealing said metal layer toform said silicide layer.
 7. The method of claim 6, wherein said metallayer is formed of a material selected from the group consisting oftungsten, titanium, cobalt, tantalum, molybdenum and platinum.
 8. Themethod of claim 1, wherein said semiconductor structure is a gate of anMOS transistor. 9-21. (canceled)
 22. A method of forming an imagingdevice having at least a gate structure with decreased impuritymigration, comprising the steps of: forming at least one photosensor ina substrate; forming a first conductive layer over said substrate;forming a capacitor structure by providing a dielectric layer over saidfirst conductive layer, and providing a second conductive layer oversaid dielectric layer; and subsequently providing a silicide layer oversaid first conductive layer and said capacitor structure.
 23. The methodof claim 22, wherein said imaging device is one of a 3T, 4T, 5T, 6T or7T imaging device.
 24. The method of claim 22, wherein said imagingdevice is a CMOS imager.
 25. The method of claim 22, wherein saidimaging device is a CCD imager.
 26. The method of claim 22, wherein saidfirst conductive layer is formed of doped polysilicon.
 27. The method ofclaim 22, wherein said second conductive layer is formed of a materialselected from the group consisting of a polysilicon, polysilicon/WSi andpolysilicon/WN/W.
 28. The method of claim 22, wherein said secondconductive layer is formed of polysilicon.
 29. The method of claim 28,wherein said second conductive layer is formed by deposition at atemperature between about 600° C. to about 800° C.
 30. The method ofclaim 22, wherein said step of providing said silicide layer includesproviding a metal layer and annealing said metal layer to form saidsilicide layer.
 31. The method of claim 22, wherein said dielectriclayer is formed of a material selected from the group consisting of anoxide, nitride, Al2O3, Ta2O5, and BST.
 32. A DRAM, comprising: at leasta gate structure formed over a substrate, said gate structure comprisinga first conductive layer and a first region of a silicide layer oversaid first conductive layer; and at least one capacitor adjacent saidgate structure, said capacitor comprising a first capacitor electrode, adielectric layer, a second capacitor electrode and a second region ofsaid silicide layer over said second capacitor electrode.
 33. The DRAMof claim 32, wherein said gate structure further comprises a gate oxidelayer.
 34. The DRAM of claim 32, wherein said first conductive layer isformed of doped polysilicon.
 35. The DRAM of claim 32, wherein saidfirst capacitor electrode is formed of doped polysilicon.
 36. The DRAMof claim 32, wherein said second capacitor electrode is formed of dopedpolysilicon.
 37. The DRAM of claim 32, wherein said dielectric layer isformed of a material selected from the group consisting of an oxide,nitride, Al2O3, Ta2O5, and BST.
 38. An imaging device, comprising: asubstrate; at least one photosensor in said substrate; at least onein-pixel gate structure formed in an active region of said substrate,said at least one in-pixel gate structure comprising a first conductivelayer and a first region of a silicide layer over said first conductivelayer; and at least one capacitor formed over an isolation regionadjacent said active region and a peripheral region of said substrate,said at least one capacitor comprising a first capacitor electrode, adielectric layer, a second capacitor electrode and a second region ofsaid silicide layer over said second capacitor electrode.
 39. Theimaging device of claim 38, wherein said at least one in-pixel gatestructure further comprises a gate oxide layer.
 40. The imaging deviceof claim 38, wherein said first conductive layer is formed of dopedpolysilicon.
 41. The imaging device of claim 38, wherein said firstcapacitor electrode is formed of doped polysilicon.
 42. The imagingdevice of claim 38, wherein said second capacitor electrode is formed ofundoped polysilicon.
 43. The imaging device of claim 38, wherein saiddielectric layer is formed of a material selected from the groupconsisting of an oxide, nitride, Al2O3, Ta2O5, and BST.
 44. A CMOSimager system, comprising: (i) a processor; and (ii) a CMOS imagingdevice coupled to said processor, said CMOS imaging device comprising: asemiconductor substrate; at least one photosensor in said substrate; atleast one gate structure formed in an array region of said substrate,said at least one gate structure comprising a first conductive layer anda first region of a silicide layer over said first conductive layer; andat least one capacitor adjacent said gate structure, said at least onecapacitor comprising a first capacitor electrode, a dielectric layer, asecond capacitor electrode and a second region of said silicide layerover said second capacitor electrode.
 45. A CCD imaging device,comprising: a semiconductor substrate; at least one photosensor in saidsemiconductor substrate; at least one in-pixel gate structure formed inan active region of said substrate, said at least one in-pixel gatestructure comprising a first conductive layer and a first region of asilicide layer over said first conductive layer; and at least onecapacitor formed over an isolation region, said isolation region beingadjacent said active region and a peripheral region of said substrate,said at least one capacitor comprising a first capacitor electrode, adielectric layer, a second capacitor electrode and a second region ofsaid silicide layer over said second capacitor electrode.